1. Field of the Invention
The present invention relates to an antenna unit and a receiver.
2. Description of the Related Art
Digital audio broadcasting in the U.S. is called DARS (Digital Audio Radio Service). To realize stable reception in a receiver mounted in a vehicle, DARS uses both satellite waves and terrestrial waves.
That is, DARS uses the 2.3 GHz band. As shown in part B in FIG. 8, two services are transmitted. In this instance, each of the services uses a frequency bandwidth of 12.5 MHz. As shown in part A in FIG. 8, one service comprises two ensembles A and B. Each of the ensembles A and B provides 50 channels of programs (contents). Accordingly, one service provides 100 channels of programs.
The ensemble A is transmitted by signals A1, A2, and A3. The ensemble B is transmitted by signals B1, B2, and B3. That is, the contents of the signals A1, A2, and A3 are identical to each other. The contents of the signals B1, B2, and B3 are also identical to each other. Consequently, when any one of the signals A1, A2, and A3 can be received, the programs of the ensemble A can be received. Similarly, when any one of the signals B1, B2, and B3 can be received, the programs of the ensemble B can be received.
As shown in part A in FIG. 8, the signals A1 to A3 and the signals B1 to B3 are arranged in order of frequency, so that the signals A1, A2, and A3 and the signals B3, B2, and B1 are symmetrically arranged with respect to the central frequency fc between the signals A3 and B3.
The signals A1, A2, B1, and B2 are QPSK (Quadrature Phase Shift Keying) signals. The signals A1 and B1 are transmitted from a broadcasting satellite BS1 located over the Western U.S. and the signals A2 and B2 are transmitted from a broadcasting satellite BS2 located over the Eastern U.S. (strictly, the satellites BS1 and BS2 are positioned on the equator at a longitude corresponding to the Western and Eastern U.S.). The signals A3 and B3 are OFDM (Orthogonal Frequency Division Multiplex) signals and are transmitted from an antenna on the ground.
Therefore, since the signals A1, A2, B1, and B2 are satellite waves and a diversity effect is obtained by the satellites BS1 and BS2, the broadcasts can be received all over the U.S. In some cases, high-rise buildings block radio waves. The terrestrial-wave signals A3 and B3 compensate to the blocked waves. Consequently, in the receiver mounted in a car, even when the radio waves vary strongly due to the motion of the car, the broadcasts can still be easily received.
When the above-mentioned DARS signals are received by the receiver mounted in the car, a receiving antenna thereof has low directivity in order to obtain uniform sensitivity irrespective of the direction of travel of the car. However, the gain of a low-directivity receiving antenna is small.
Consequently, the reception levels of the signals A1, A2, B1, and B2 transmitted from the satellites BS1 and BS2 are considerably low. Actually, the reception levels of the signals A1 to B2 are higher than the noise level of the receiving antenna by 10 dB to 20 dB; that is, the levels are substantially equal to −100 dBm to 90 dBm. When the DARS signals are received, therefore, a high-frequency amplifier for amplifying the output of the receiving antenna is needed. In addition, a high-frequency amplifier with sufficiently low noise is needed.
On the other hand, the reception levels of the signals A3 and B3 transmitted from the antenna on the ground vary strongly depending on the distance from the transmitting antenna to the receiving antenna, namely, the levels are substantially equal to −90 dBm to 0 dBm. When the signals A3 and B3 are received near the transmitting antenna, the reception levels rise considerably, so that the high-frequency amplifier becomes saturated, causing large distortion.
Consequently, in consideration of the above description, a high-frequency amplifier which has a low noise level and small distortion even when the input level varies over a range of 100 dB is needed at an antenna input stage of the receiver.
Generally, when the reception level varies, AGC (Automatic Gain Control) is performed to stabilize the signal level. In case of a DARS receiver, the AGC must respond to a variable range of the reception level as high as 100 dB.
Furthermore, when the DARS receiver is mounted in the car, an antenna thereof is disposed on a portion, in which reception interference is small, for example, on the roof. Since the receiver is arranged in the car, the antenna is connected to the receiver through a cable. As mentioned above, however, the DARS broadcasts use the 2.3 GHz band, so attenuation caused by the cable is large. Generally, attenuation of about 10 dB is caused. Accordingly, when the antenna is simply connected to the receiver through the cable, it is difficult to receive the satellite-wave signals A1 to B2.
Generally, in such a case, a high-frequency amplifier is integrated with the antenna, a received signal of the antenna is amplified by the high-frequency amplifier, and the resultant signal is supplied to the receiver through the cable. In this instance, the necessary operating voltage for the high-frequency amplifier is supplied from the receiver through the cable.
However, to perform AGC in the high-frequency amplifier integrated with the antenna, an AGC voltage is generated from the receiver, thus necessitating a line for supplying the AGC voltage to the high-frequency amplifier. Consequently, a special cable or connector is needed.
To perform AGC in the high-frequency amplifier, the high-frequency amplifier should comprise a variable gain amplifier. The NF (Noise Figure) of the variable gain amplifier is generally lower than that of a fixed gain amplifier. Accordingly, the variable gain amplifier cannot be used as a high-frequency amplifier in which low noise is required.
The present invention intends to solve the above problems.